Mysteries of Modern Physics: Time

Studying the physics of time; the meaning of a clock; understanding a phrase; the arrow of time; the concept of entropy; how it holds the key to the one-way direction of time in the universe.

Approaching time from a philosophical perspective; presentism holds that the past and future are not real; only the present moment is real; the laws of physics appear to support eternalism that says all moments in the universe are equally real.

How to measure the passage of time; the practical concerns have driven the search for more and more accurate clocks; the problem of longitude is solved with a timepiece of unprecedented accuracy; GPS navigation units rely on accurate clocks.

Understanding time; the arrow of time; how memory and ageing are related to time; looking at irreversible processes, such as an egg breaking or ice melting; the essence of the one-way direction of time.

The history of the second law of thermodynamics; it is considered by many physicists to be the one law of physics most likely to survive unaltered for the next thousand years; entropy is the degree of disorder in a closed system.

Isaac Newton's laws of physics are fully reversible; particles can move forward or backward in time without any inconsistency; the arrow of time is fundamentally connected to irreversible processes and the increase in entropy.

Advances in physics reveal exceptions to the rule that interactions between moving particles are fully reversible; irreversible reactions between elementary particles explain the arrow of time; the evidence for and against this view.

The theory of quantum mechanics; it leads to startling interpretations of the nature of reality; a quantum state called the collapse of the wave function may underlie the arrow of time.

Establishing that the arrow of time must be due to entropy; an exploration of this phenomenon; physicist Ludwig Boltzmann proposes a definition of entropy that explains why it increases toward the future; analyzing the idea.

Understanding entropy; examining different macroscopic systems; high entropy and low entropy; James Clerk Maxwell designs a thought experiment about a demon who seemingly defies the principle that entropy always increases.

Boltzmann explains why entropy will be larger in the future; he does not show how it was smaller in the past; physics cannot account for this difference except by assuming that the universe started in a state of very low entropy; the past hypothesis.

Wondering if physics can shed light on human aspects of the arrow of time such as memory, cause and effect and free will; everyday features of experience trace back to the low entropy state of the universe at the Big Bang.

One possible explanation for order in the universe is that it is a random fluctuation from a disordered state; whether the universe is one such fluctuation that is in the process of returning to disorder; investigating Boltzmann brains.

Maxwell's demon provides the key to understanding how complexity and life can exist in a universe in which entropy is increasing; how life is not only compatible with, but is an outgrowth of, the second law of thermodynamics and the arrow of time.

The way in which humans perceive time; how it varies from clock time; experiments that shine light on the sense of time; tests demonstrate how humans perceive the present moment but live 80 milliseconds in the past.

Remembering the past and projecting into the future are crucial for human consciousness, as shown by cases where these faculties are impaired; investigating how memory works; exploring different kinds of memory; the phenomena of false memories.

Einstein's theory of relativity; there is no such thing as a moment in time spread throughout the universe; time is one of four dimensions in spacetime; how this relative view of time is usefully diagrammed with light cones.

Einstein develops a general theory of relativity incorporating gravity; he launches a revolution in the understanding of the universe; how his idea that gravity results from the warping of spacetime led to the discovery of black holes.

How a time machine may work; a real time machine may be a spaceship that moves through all the intervening points between two locations in spacetime; the paradoxes of time travel.

Stephen Hawking discovers that black holes emit radiation and have entropy; entropy in the universe today is overwhelmingly in the form of black holes; there were no black holes in the early universe; entropy must have been lower in the past.

The history of the universe from just after the Big Bang to the far future; the universe may consist of virtually empty space at maximum entropy; learning about what is well-founded and what is less certain about this picture of the universe.

Three different ways of thinking about the Big Bang; the actual beginning of the universe; a bounce from a symmetric version of the universe; considering it as a region that underwent inflationary expansion in a much larger multiverse.

The possibility that the Big Bang originates in a multiverse; a plausible explanation for why entropy is low at the Big Bang; the arrow of time; how this theory and the idea of an anthropic principle may be legitimate science or science fiction.

Investigating a range of different possibilities that explain the origin of time in the universe; Carroll's favorite theories; how much remains to be done before conclusively solving the mystery of time.